[5] mqtt,mosquitto,eclipse Paho---MQTT Message Format analysis overview

We know that MQTT is a specification of an IoT protocol, and thelatest two versions of the MQTT protocol are: 3.1.1 and 3.1.0

(1) The specifications of 3.1.0 are as follows
Http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html

(2) The specifications of 3.1.1 are as follows
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718029

Normative 3.1.0 and 3.1.1 still have a lot of difference. For example, in the variable head, in the 3.1.0 specification, the keyword is, "MQISDP"

Table 1. MQTT V3.1 Specification The first 8 bytes of a mutable message header

	Description	7	6	5	4	3	2	1	0
Protocol Name
BYTE 1	Length MSB (0)	0	0	0	0	0	0	0	0
byte 2		0	0	0	0	0	1	1	0
byte 3	' M '	0	1	0	0	1	1	0	1
byte 4	' Q '	0	1	0	1	0	0	0	1
byte 5	' I '	0	1	0	0	1	0	0	1
byte 6	' s '	0	1	1	1	0	0	1	1
byte 7	d '	0	1	1	0	0	1	0	0
BYTE 8	' P '	0	1	1	1	0	0	0	0

Table 2. MQTT V3.1.1 The first 6 bytes of a variable message header

	Describe	7	6	5	4	3	2	1	0
Agreement name
BYTE 1	Length MSB (0)	0	0	0	0	0	0	0	0
BYTE 2	Length LSB (4)	0	0	0	0	0	1	0	0
BYTE 3	M	0	1	0	0	1	1	0	1
BYTE 4	' Q '	0	1	0	1	0	0	0	1
BYTE 5	T	0	1	0	1	0	1	0	0
BYTE 6	T	0	1	0	1	0	1	0	0
Protocol level
	Describe	7	6	5	4	3	2	1	0
BYTE 7	Level (4)	0	0	0	0	0	1	0	0

The specification mainly describes the general format of the message, the command of the message, the flow of messages, and so on. In the previous four chapters, we have a perceptual understanding of MQTT, and in the next series of articles, I will take you to the following 14 kinds of command messages for MQTT through the capture package software (WireShark) and combine the specific specifications with everyone else. Note that because I use the Java Library of the Eclipse Paho API to support the version of Mqtt 3.1.1, and Mosquitto also supports the 3.1.1 version, all of the capture packages in this series will be based on the Mqtt 3.1.1 specification, not the MQTT 3.1 specification. Please pay attention to them. The following table describes the different message types in Mqtt 14.

Table 3. 14 Message types for MQTT (Command message)

Name	Value		Direction of information flow	Describe
Reserved	0	Forbidden		Reserved
CONNECT	1	Client to Server		Client request to connect to Server
CONNACK	2	Server to Client		Connect Acknowledgment
PUBLISH	3	Client to Server Or Server to Client		Publish message
Puback	4	Client to Server Or Server to Client		Publish Acknowledgment
Pubrec	5	Client to Server Or Server to Client		Publish received (assured Delivery Part 1)
Pubrel	6	Client to Server Or Server to Client		Publish release (assured Delivery Part 2)
Pubcomp	7	Client to Server Or Server to Client		Publish complete (Assured Delivery Part 3)
SUBSCRIBE	8	Client to Server		Client Subscribe Request
SUBACK	9	Server to Client		Subscribe Acknowledgment
Unsubscribe	10	Client to Server		Unsubscribe Request
Unsuback	11	Server to Client		Unsubscribe Acknowledgment
PINGREQ	12	Client to Server		PING Request
Pingresp	13	Server to Client		PING response
DISCONNECT	14	Client to Server		Client is disconnecting
Reserved	15	Forbidden		Reserved

In the following chapters we will be one to everyone with Wireshark grab bag, and give everyone an analysis of their agreement, please look forward to and attention, thank you.

[5] mqtt,mosquitto,eclipse Paho---MQTT Message Format analysis overview